1. Field of the Invention
This invention relates to active matrix addressing and methods of construction for surface deformation type spatial light modulators.
2. Prior Art
Surface deformation type wavefront modulators defines a broad class of devices, including membrane light modulators. Previous membrane light modulators have been hindered by not having a reliable, cost effective, high speed, active matrix addressing scheme.
Several membrane modulators have been electron beam addressed. Examples include the device described in the article "Television: A New Schlieren Light Value for Television Projection" by J. A. van Raalte, Applied Optics, October 1970, Vol. 0, NO. 10. This configuration suffers from a poor luminous efficiency. No charge storage exists, and charge decay is utilized to erase an image. As identified in the article, dirt and/or dust contamination of the membrane is a problem that needs to be addressed. Further complications associated with electron beam addressing include packaging, high voltage requirements, and a vacuum environment. Similar limitations exist in U.S. Pat. No. 2,910,532 to M. Auphan, Oct. 27, 1959, and U.S. Pat. No. 2,681,423, to M. Auphan, Jun. 15, 1954. Auphan identifies several different reflective deformable conductive embodiments in U.S. Pat. No. 2,681,244. U.S. Pat. No. 2,681,244 suffers from several complications including electron beam addressing and reduced luminous efficiency due to the resistivity of the insulating substrate serving as a discharge mechanism for the capacitive membrane elements. Although several different embodiments are common to U.S. Pat. No. 2,681,244, as to be shown herein, the embodiments of U.S. Pat. No. 2,681,244 are not equivalent.
As identified in U.S. Pat. No. 2,681,423, one of the objectives was to present a light reflecting screen consisting of strips which are not insulated from one another. As to be shown herein, such an electrical arrangement between column electrodes restricts the latitude available in configuring an active matrix array of thin film transistors. As to be shown herein, this aspect has not been previously appreciated by those knowledgeable in the state of the art. Furthermore, my invention teaches away from prior art.
Additional electron beam addressed membrane light modulators include U.S. Pat. No. 3,701,586, to G. G. Goetz, Oct. 31, 1972, U.S. Pat. No. 3,746,785 to Goodrich, Jul. 17, 1973, and the configurations described in the articles "Experimental Parameters of the Photoemitter Membrane Spatial Light Modulator", by Peter B. Rolsma et al, Applied Optics/Vol. 28, No. 22/15 November 1989, "Photoemitter Membrane Light Modulator" by Arthur D. Fisher et al, Optical Engineering/February 1986/Vol. 25 No. 2, and "The Photoemitter-Membrane Light Modulator Image Transducer" by L. E. Sommers, Advances in Electronics and Electron Physics, Vol. 33A, 1972. Complications of a vacuum environment and impact on the membrane are provided in the cited references.
Additional membrane light modulator configurations exist and include optically addressed membrane light modulators such as U.S. Pat. No. 3,463,572 to K. Preston, Jr. Nov. 18, 1969. As well understood by those knowledgeable in the state of the art, optical addressing of light values employed in electronic to optical data conversion applications, such as HDTV and Infrared scene projection, could adversely effect signal fidelity due to unnecessary signal transformations required to optically address such a device. Accordingly, U.S. Pat. No. 3,463,572 is deemed undesirable for use in electronic to optical data conversion applications.
A "passive", coincidence matrix addressing schemes is employed in U.S. Pat. No. 4,001,635 to d'Auria et al, Jan. 4, 1977. Several of the problems associated with passive matrix schemes can be found in the article "A 6.times.6-in 20-lpi Electroluminescent Display Panel" by T. P. Brody et al, IEEE Transactions on Electron Devices, Vol. ED-22, No. 9, September 1975. Furthermore, coincident matrix addressing hinders high frame rate capability due to each pixel being individually addressed.
U.S. Pat. No. 4,694,287 to Chenevas-Paule et al, Sep. 15, 1987, describes an active matrix addressing scheme for use with liquid crystal media. In U.S. Pat. No. 4,694,387 column addressing electrodes are affixed to "walls" of the device. Utilizing transmissive column electrodes affixed to "walls" of the device precludes any consideration for U.S. Pat. No. 4,694,287 to function as a membrane light modulator. If it is contemplated to utilize the electrode structure of U.S. Pat. No. 4,694,287 in a membrane light modulator configuration, additional components must be added. Additional components required so that U.S. Pat. No. 4,694,287 could be configured to function as a membrane light modulator would include reflective deformable conductors. Several surface deformation type modulators have utilized conductors affixed to "walls" to establish a potential difference across the deformable media. Examples include U.S. Pat. No. 3,626,084 to Whol, Dec. 7, 1971 and the device described in the article "Deformable Surface Spatial Light Modulator" by K. Hess et al, Optical Engineering/May 1987/Vol. 26, No. 5. Quoting from the article, "The gel elastomer (3) is placed between a transparent electrode (2) and a comb-shaped electrode (7). The application of a supply voltage (typically 200 to 300 V rms) between the two electrodes produces a nonuniform electric field, which deforms the dielectric elastomer and results in a gratinglike deformation of the free gel surface nearest the air gap (4)." This emphasizes that for surface deformation type modulators, a gap will exist between conductors affixed to the "walls", and the deformable media. "Walls" utilized to support conductors for use with deformable media could be viewed as extraneous components, hindering performance in several respects. Deformation type modulators utilizing conductors affixed to "walls" suffer from capactive voltage division across the gap between the deformable media and the conductors affixed to the "wall". As to be shown herein, gaps in membrane modulators could be eliminated. Gaps reduce modulation efficiency, since higher voltages must be utilized to establish a required deformation when compared to the case of no gap. This could adversely effect the geometry of switches utilized in an active matrix, since each switching element must handle a correspondingly higher voltage. This could adversely effect spatial resolution. Accordingly, U.S. Pat. No. 4,694,287 is undesirable for use with surface deformation type spatial light modulators. It should be noted, later configurations of the Deforgraphic Storage Display Tube employed a conductive electrode affixed to the deformable medium; see for instance U.S. Pat. No. 3,879,630 to Halperin et al, Apr. 22, 1975, and Technical Report RADC-TR-71, "Dielectric Membrane Light Value Study". Presumably, these later configurations evolved from the realization that elimination of the gap would enhance efficiency.
In addition to the reduced modulation efficiency attributed to capacitive voltage division, if reflective deformable conductors are added to U.S. Pat. No. 4,694,287 so the electrode configuration of U.S. Pat. No. 4,694,287 could be utilized in a membrane light modulator configuration, additional inefficiencies would exist. This is attributed to the face that each electrode labeled as "E" in U.S. Pat. No. 4,694,287 and a respective transistor are affixed to the same surface of a substrate. This prevents the electrodes "E" from conveniently overlapping a respective transistor. Consequently, the percentage of area that an electrode "E" occupies of an electrode "E" and respective transistor combination will be less than if each electrode "E" could overlap a respective transistor, ie the percentage of area that an electrode occupies of the image point area is reduced when compared to a case where the electrode can overlap a transistor.
As identified in the article "A Membrane Page Composer" by L. S. Cosentino and W. C. Stewart, RCA review Vol. 34, March 1973, reducing the percentage of the spatial period occupied by an electrode could reduce the modulation efficiency of the membrane light modulator.
As to be identified herein, my invention could utilize both major opposing surfaces of a substrate to allow each electrode to overlap a respective transistor thereby enhancing the modulator efficiency while eliminating extraneous components, such as a second substrate, when compared to prior art.
As to be inferred from the evolution of the Deforgraphic Storage Display Tube, considerations to enhance modulator efficiency are not obvious.
Since U.S. Pat. No. 4,694,287 would require additional components to function as a membrane light modulator and addition of such components yields a configuration that suffers from several complications, U.S. Pat. No. 4,694,287 is deemed unsuitable for use as a membrane light modulator.
U.S. Pat. No. 4,441,791 to Hornbeck, Apr. 10, 1984 describes a deformable mirror device disposed over a semiconductor substrate. Configurations of this nature are complicated. This statement is substantiated by the comments made in the article "Micromechanical Light Modulators on Silicon" by Robert E. Brooks, Optical Engineering, January February 1985, Vol. 24, No. 1. As quoted from the article, "These devices are not simple to make and require fabrication steps that are not completely compatible with integrated circuit (IC) processing methods."
In addition, the fabrication approach taken in U.S. Pat. No. 4,441,791 hinders achieving performance capabilities inherent in membrane elements. Speed of response is an important consideration for several applications, including infrared scene projection. Frame rates as high as a 1000 HZ are desired. Selection of a particular semiconductor substrate could limit alternatives available for interface components. CCD implementations in silicon have performance limitations when compared to GaAs Schottky barrier gate shift registers. Speed limitations of U.S. Pat. No. 4,441,791 are evident by the circuit configurations integrated with the device. For instance, to increase the data transfer rate, it is suggested to split the shift registers. This approach complicates construction and falls far short of the performance capabilities of alternatives. U.S. Pat. No. 4,441,791 utilizes electrical busses fabricated in a doped semiconductor and this could adversely effect speed of response of interface circuitry. Furthermore, use of a semiconductor substrate could limit the dimensions available for a modulator array. This could hinder implementation of large format devices, and/or the use of hybrid interface circuits to enhance performance capabilities, such as high speed shift registers.
As previously cited, contamination problems could render a membrane device ineffective. Having a membrane light modulator constructed as a monolithic device could adversely effect profits associated with a device exhibiting a poor yield. Construction of a device should allow for testing at an early stage of fabrication, involving reduced circuit complexities, to identify array problems prior to full scale system integration. This procedure would reduce the impact of poor yield. Such procedures are difficult to implement with devices constructed similar to U.S. Pat. No. 4,441,791. In addition, the complexity of the support grid in U.S. Pat. No. 4,441,791 hinders cleaning operations prior to affixing the membrane to it's support structure. Contamination and it's complications has hindered significant commercialization of this device. See for instance the comments concerning susceptibility to defects of membrane light modulators in U.S. Pat. No. 4,956,619 to Hornbeck, Sep. 11, 1990 and U.S. Pat. No. 4,710,732, to Hornbeck, Dec. 1, 1987. The fabrication sequence of U.S. Pat. No. 4,441,791 exposes the entire integrated system to contamination problems associated with the membrane. This risk is undesirable and as to be shown herein, unnecessary. In addition, by utilizing front side interface electronics, the substrate cannot serve to shield light sensitive components in this device. Poor utilization of system resources requires extraneous components to perform this function, increasing device investment prior to performance evaluations, and complicating device structure.
An alternative construction for a membrane modulator is described in U.S. Pat. No. 3,798,620 to Cosentino, Mar. 19, 1974. In this patent, crossovers are incorporated into the discrete transistor modules which are bonded to the backside of the device. As to be show herein, by suitable utilization of the membrane, requirements for crossovers in the switching array can be eliminated, not just translated as a requirement for another interface component. In addition, bonding several transistors to the substrate prior to membrane checkout is undesirable; the number of fabrication operations should be reduced prior to membrane checkout.
In addition, transferring the electrical crossover requirement to the semiconductor interface component hinders integration of alternative switching elements since the semiconductor interface component must have a packaging configuration which can accommodate the electrical crossover conductor. As to be shown herein, my invention utilizes column conductors to eliminate the electrical crossovers networks in the semiconductor interfaces. Since my invention eliminate electrical crossover networks, semiconductor packaging requirements to accommodate electrical crossovers are extraneous requirements. As to be shown herein, eliminating extraneous packaging requirements could facilitate integration of thin film transistors with membrane light modulators.
As previously stated, the different deformable conductor embodiments of U.S. Pat. No. 2,681,423 are not equivalent. This may be demonstrated by substituting a transmissive, non deformable, monolithic electrode for the transmissive, non deformable, column electrodes in U.S. Pat. No. 4,697,284. As well understood by those knowledgeable in the state of the art, if a monolithic electrode were substituted for the column electrodes in U.S. Pat. No. 2,697,284, then a consistent mode of operation would not be maintained thereby demonstrating that the embodiments of U.S. Pat. No. 2,681,423 are not equivalent. Although the monolithic electrode and the column electrode are not deformable in the example involving U.S. Pat. No. 4,697,384, the example does serve to illustrate that monolithic and column electrode structures are, in general, not equivalent.
Since the embodiments of U.S. Pat. No. 2,681,423 to Auphan were known at the time of U.S. Pat. No. 3,798,620 to Cosentino, evidently the benefits provided by my invention were not previously appreciated by those knowledgeable in the state of the art.
As can readily be seen, several membrane light modulators configurations have evolved. However, no device has achieved significant commercialization status. This could be attributed to the complications identified in the prior art.